Unconventional interplay between heterovalent dopant elements: Switch-and-modulator band-gap engineering in (Y, Co)-Codoped CeO2 nanocrystals

Abstract

We report the experimental observation and theoretical explanation of an unconventional interplay between divalent Co and trivalent Y dopants, both of which incur oxygen vacancies in the CeO2 host that has predominantly tetravalent Ce cations. The Co dopant atoms were experimentally found to act as a switch that turns on the dormant effect of Y-modulated band-gap reduction. As revealed by density functional theory (DFT) calculations with structures verified by synchrotron-radiation x-ray measurements, a Co 3d band that hybridizes with Ce 4f band was lowered due to reduced O 2p repulsion arising from oxygen vacancies incurred by Y doping and therefore gave rise to the observed band-gap narrowing effect. Such switch-and-modulator scheme for band-gap engineering in nanocrystal materials can lead to important applications in environmental protection and solar energy harvesting technologies.

Dramatic band gap reduction incurred by dopant coordination rearrangement in Co-doped nanocrystals of CeO2

“…The band gap energy of the sample shows a drastic decrease of 1.61 eV after thermal annealing at 650 °C. It is worth noting that the band gap width of the as-made sample of Co-doped CeO 2 nanocrystals is roughly the same as that of a pure CeO 2 nanocrystal, 2.80 eV measured previously 11 . The effect of Co dopant atoms on band-gap engineering of ceria appears to be negligible compared to that of thermal annealing in the present work.…”

“…Nanocrystal samples of Co-doped CeO2 synthesized by using a polyol method 11 was annealed at temperatures 200, 300, 400, 500, 600, and 650 °C. The Co concentration of the samples was determined to be 4.3 at.…”

Formation of Co–O bonds and reversal of thermal annealing effects induced by X-ray irradiation in (Y, Co)-codoped CeO2 nanocrystals

“…Therefore, thermal annealing and X-ray irradiation can act in opposing directions to adjust the compositions of the square-planar and octahedral types of Co oxygen coordination to optimize the properties of the material for specific applications. As demonstrated in our previous works, introduction of Co dopant atoms can switch-on substantial decrease of band gap with increasing Y codopant concentration 15 and rearrangement of oxygen coordination around Co dopant atoms can lead to a dramatic band gap variation of 1.61 eV 14 in the CeO 2 host. We have thus developed a unique method to fine tune the band-gap-related electronic and optical properties of (Y, Co)-codoped CeO 2 nanocrystals using thermal annealing and X-ray irradiation for various technological applications.…”

“…Samples of (Y, Co)-codoped CeO 2 nanocrystals were synthesized using a polyol method 15 . The as-grown samples were then annealed under pure O 2 at 300 °C for 30 min with a ramping rate of 10 °C/min such that the square-planar oxygen coordination is dominant surrounding the Co dopant atoms 14 .…”

Exsolution of Metallic Ru Nanoparticles from Defective, Fluorite-Type Solid Solutions Sm2RuxCe2–xO7 To Impart Stability on Dry Reforming Catalysts

“…The Ru K-edge EXAFS fitting results of Sm 2 Ru x Ce 2– x O 7 (Table S2) show an average coordination number of the nearest Ru–O shell at 1.97(1) Å of 6(1). The fact that this coordination number is lower than the metal–oxygen coordination in a perfect nondefective fluorite-type structure (8 oxygen neighbors) and lower than for the Ce–O and Sm–O paths (vide infra) suggests that the oxygen vacancies in the solid solution are preferentially located adjacent to the Ru cations. , Moreover, this preferential location of oxygen vacancies in the vicinity of Ru atoms, together with the differences between the Ru–O distance and the longer Ce–O and Sm–O distances (Table S3 and vide infra), leads to a local, structural distortion around the Ru atoms. The local disorder around Ru in the Sm 2 Ru x Ce 2– x O 7 solid solutions leads in turn to an unobservable second shell by Ru K-edge EXAFS (Figure c,d).…”